In a closed vessel, the tank is sealed, which allows for a vapor pressure above the process fluid. In an open tank, the vessel is open to atmospheric pressure.

The level is monitored at all times

Direct measurements directly measure the level, while indirect measurements inferred level from other measured parameters.

Boundary between two immiscible (incapable of being mixed) fluids

The level is measured as being between several discrete points.

Non-Contact measurements have to direct contact with the process fluid while contact measurements do.

The level is measured as either above or below a singular point.

Top-down measurements measure level from above the process fluid, while bottom-up measurements measure from below the process fluid.